1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker""s fabric of the on-machine-seamable variety, such as an on-machine-seamable press fabric for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the fibers in the web to one another to turn the fibrous web into a sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The web, or newly formed paper sheet, itself is directed in a sinuous path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a paper machine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
In brief, the seam region of any workable on-machine-seamable, or OMS(copyright), press fabric must behave under load, that is, under compression in the press nip or nips, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the periodic marking of the paper product being manufactured by the seam region. OMS(copyright) is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable press fabric, because of the comparative ease and safety with which it could be installed on the press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an on-machine-seamable press fabric, than it is to install an endless press fabric, on a paper machine.
There are several methods for producing a press fabric that can be joined on the paper machine with such a seam. One method is to flat-weave the fabric, in which case the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric. In still another method, a fabric is woven endless, and the endless loop of fabric thereby obtained is flattened and given the form of two fabric layers joined to one another at two widthwise ends of the flattened loop. One or more widthwise yarns are then removed from each of the two widthwise ends to produce a short gap defined by the freed, that is, the newly unwoven portions of, lengthwise yarns at each end. These unwoven portions of the lengthwise yarns are then used as seaming loops when the two widthwise ends are brought together as described above.
Alternate approaches toward manufacturing a laminated fabric like that made according to the preceding method are shown in two recently issued U.S. patents. In commonly assigned U.S. Pat. No. 5,732,749 to Fargeout, the teachings of which are incorporated herein by reference, a laminated integrally woven on-machine-seamable papermaker""s fabric includes two single-layer woven fabric plies sharing a common machine-direction (MD) yarn. The common MD yarn, which is the weft yarn on the loom during the weaving of the fabric by a modified endless weaving technique, forms seaming loops which join the plies to one another at the ends thereof. During the weaving of the press fabric, solvent-removable binder yarns join the two plies, which are accordingly integrally woven. When the weaving is completed, the solvent-removable binder yarn are removed through dissolution with an appropriate solvent, separating the plies and yielding the laminated structure.
In commonly assigned U.S. Pat. No. 5,939,176, the teachings of which are incorporated herein by reference, an on-machine-seamable multiaxial press fabric for the press section of a paper machine is made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless base fabric layer is flattened to produce first and second fabric plies joined to one another at folds along their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at the folds to produce seaming loops. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops.
Generally, the manufacture of an on-machine-seamable press fabric includes the attachment of a staple fiber batt to one or both sides thereof. The attachment may be effected by a process called needling (fiber locking) or hydroentangling, while the fabric is joined into endless form. Once the desired amount of staple fiber batt has been attached, the loop-forming pin or pintle is removed to place the press fabric into flat form for shipment and eventual installation on a paper machine. At this time, the staple fiber batt must be cut in the vicinity of the seam to completely separate the two ends of the press fabric from one another. Often, the staple fiber batt is cut in a manner that enables it to form a flap over the seaming loops when the press fabric is rejoined into endless form. In this way, the seam region is practically indistinguishable from the rest of the paper-supporting side of the press fabric.
On the other side, the xe2x80x9crollxe2x80x9d side, of the press fabric, however, some staple fiber batt must be removed from the seaming loops to facilitate the later passage of a pintle therethrough. The removal of this generally small amount of staple fiber batt, nevertheless, makes the seam region slightly more permeable to air and water than the rest of the press fabric. This difference in water permeability, or flow resistance, perhaps ever so slight, is enough to cause sheet marking in some situations. Moreover, the difference tends to become more pronounced with time as more batt fiber is lost from the xe2x80x9crollxe2x80x9d side of the press fabric as it runs on the paper machine.
Several approaches to solve this problem have been taken. One approach involves the use of stuffer yarns with the pintle when the press fabric is being joined into endless form on the paper machine.
In another approach, a press fabric comprises two on-machine-seamable base fabrics, one fitting inside the loop of the other, laminated to one another during the needling process. The seam regions of the inner and outer base fabrics are offset slightly with respect to one another, so that the seam region of each will coincide with a non-seam region of the other.
All of these approaches are designed to compensate for the differences between the water permeability or flow resistance of the seam region of an OMS(copyright) press fabric and the rest or body of the press fabric. However, none of these approaches have yielded completely satisfactory results for all press types and positions and for all paper grades.
The present invention represents an alternative approach toward solving this problem.
Accordingly, the present invention is an on-machine-seamable papermaker""s fabric having flow-resistant material applied over or adjacent to its seam to compensate for differences between the permeabilities to air and water of the seam and of the rest of the papermaker""s fabric.
In a first embodiment of the papermaker""s fabric, an on-machine-seamable base fabric has a system of longitudinal yarns and a system of transverse yarns, the former being substantially in the machine direction and the latter being substantially in the cross-machine direction. The yarns of the system of longitudinal yarns are interwoven with the yarns of the system of transverse yarns to form the base fabric in a rectangular shape with a length, a width, two lengthwise edges and two widthwise edges. The longitudinal yarns form seaming loops along each of the two widthwise edges. The seaming loops are interdigitated and joined together with a pintle to form a seam.
A strip of flow-resistant material overlies the seam and is attached thereto. The flow-resistant material provides the seam with permeabilities to air and water substantially identical to that of the rest of the papermaker""s fabric. At least one layer of staple fiber batt is entangled through the base fabric.
In an alternate embodiment of the invention, the on-machine seamable papermaker""s fabric has a base fabric with a laminated structure of a top layer and a bottom layer. These two layers are joined directly to one another only at their two widthwise edges by longitudinal yarns, which form seaming loops there. As above, a strip of flow-resistant material may be placed over the seam formed when the seaming loops are interdigitated with one another and joined together with a pintle. Alternatively, strips of flow-resistant material may be placed between the top layer and the bottom layer adjacent to and on each side of the seam. As above, the flow-resistant material provides the seam with permeabilities to air and water substantially identical to that of the rest of the papermaker""s fabric. At least one layer of staple fiber batt is entangled through the base fabric to join the two layers to one another.
The present invention will now be described in more complete detail with frequent reference being made to the drawing figures identified below.